1. Field of the Invention
The present invention relates to a back light structure of a liquid crystal display device, and particularly, to a direct type back light structure of a liquid crystal display that provides an improved contrast ratio.
2. Description of the Related Art
Because of the recent developments in various portable electronic devices, such as mobile phones, PDAs and notebook computers, the demand for a light, thin, small flat panel display device is increasing. Research is actively ongoing on several types of flat panel display devices including LCD (Liquid Crystal Display), PDP (Plasma Display Panel), FED (Field Emission Display), VFD (Vacuum Fluore scent Display) and other technologies. Of the aforementioned flat panel display devices, the LCD has received much attention because it is simple to mass produce and can be easily used with a driving system that implements a high quality picture.
The LCD device is a transmissive display device. More particularly, an LCD device displays a desired image by controlling the amount of light transmitted through a liquid crystal layer by using the refractive anisotropy of the liquid crystal molecules in the liquid crystal layer. A back light is on the LCD device and used as a light source for light transmitted through a liquid crystal layer to display an image.
In general, there are two types of back lights. The first type of back light is the edge type back light in which a lamp is at an edge of an LCD panel for providing light to the liquid crystal layer. The second type of back light is the direct type back light in which a lamp is positioned directly under the LCD panel for providing light to the liquid crystal layer.
The edge type back light includes a low profile reflector and a light guide positioned directly under the LCD panel. The edge type back light is commonly used in a notebook computer or like devices which require a thin display device. However, the edge type back light has problems. For example, the edge type back light is not suitable for a large LCD panel because the lamp positioned at the edge of the LCD panel can not provide enough light across a large LCD panel. In another example, high brightness is difficult to obtain because the light is supplied through a light guide, which causes light loss. Thus, an edge type back light is only suitable for a small LCD panel. In other words, an edge type back light would not be desirable for a large LCD TV display.
The direct type back light can be used in a large LCD panel because the light generated from the lamp or lamps of a direct back light is supplied directly to the liquid crystal layer. Thus, light of high brightness can be obtained uniformly across the LCD panel. Thus, the direct type back light is typically being used to fabricate an LCD panel for an LCD TV display.
FIG. 1 shows a structure of an LCD device using the direct type back light in accordance with the related art. As shown in FIG. 1, the LCD device 1 includes an LCD panel 3; and a back light 10 at a back surface of the LCD panel 3. The LCD panel 3 is where an image is actually displayed. The LCD panel includes a transparent lower substrate 3a and an upper substrate 3b, such as glass, and a liquid crystal layer (not shown) formed between the lower and upper substrates 3a and 3b. Although not shown in FIG. 1, the lower substrate 3a is a TFT substrate having driving structures, such as thin film transistors and pixel electrodes, and the upper substrate 3b is a color filter substrate having a color filter layer. In addition, a driving circuit unit 5 is provided at a side of the lower substrate 3a to apply a signal to the pixel electrodes via the thin film transistors.
The back light 10 includes a lamp 11 for emitting light; a reflector 17 for reflecting light emitted from the lamp 11 to improve optical efficiency; and an optical elements for making the light, which is emitted from the lamp 11, to be uniformly incident upon an LCD panel 3. The optical elements of the direct type back light 10 are commonly a diffuser sheet 15 and a diffuser plate 16. The diffuser sheet 15 and diffuser plate 16 scatter light generated from the lamp 11 so that light is uniformly distributed across the LCD panel 3. However, there is a limit to the amount of light that can be uniformly distributed across the LCD panel. In addition, because the diffuser sheet 16 diffuses the incident light omnidirectionally, a lot of light diffuses out the sides, which causes degradation in optical efficiency. If the diffuser sheet 16 is disposed on the diffuser plate 15, side scattering of light is prevented. However, in such a case, back scattering of the diffuser plate 15 and the diffuser sheet 16 occurs, thereby degrading the optical efficiency, and an internal temperature of the back light 10 increases. Moreover, to improve straightness of light emitted from the back light 10, a prism sheet may be employed. However, such a prism sheet causes an increase in cost, and, substantially, there is limit to how much the straightness of light can be improved with the prism sheet.